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Vitamin D is the new kid on the block. Long neglected andthought of only as a bone vitamin that was needed to regulate calcium metabolism, it is now known to be involved in many more biological reactions. The most significant finding in recent years was that vitamin D3, or calciferol, was not the active form of vitamin D. That honor belongs to the molecule known by the chemical name 1 alpha, 25 dihydroxyvitamin D3.

The chemistry of vitamin D

Vitamin D is derived from the cholesterol molecule. In the skin, cholesterol is converted to a new form called 7-dehydrocholesterol, which reacts with ultraviolet B (UVB) light at wavelengths between 270–300 nm (peak 295–297 nm). These wavelengths are found in sunlight when the UV index is greater than 8–9<sup>a</sup>. In temperate regions during the spring and summer, adequate amounts of vitamin D3 as the provitamin D can be made in the skin with only 10–15 minutes of sun exposure at least twice a week. Although not recommended, exposing the face, arms, hands or back without sunscreen makes enough vitamin D to meet the daily requirement of the body. If you make more vitamin D by being in the sun for longer periods, the body will simply destroy it. You can see the cholesterol molecule in Figure 1.

The synthesis of vitamin D3

The only active form of vitamin D is the 1 α, 25, dihydroxy D3. There are two starting molecules from which active vitamin D can be synthesized. One form is called cholecalciferol, which is formed directly from 7-dehydrocholesterol and is derived from cholesterol. See Figure 1 for a clear illustration. The second starting material is ergocalciferol, called vitamin D2, which is formed from ergosterol, a compound that is very similar to cholesterol that occurs in plants and fungi.

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In the skin, newly made cholecalciferol is sent to the liver where it is converted to 25 hydroxy vitamin D3 by enzymes in the mitochondria, which is the first essential step in making active vitamin D. When you see or use the word “vitamin D” it will mean 1 α, 25, dihydroxy D3. Now it must have one more hydroxy group (OH) attached at the number one carbon in the alpha position. This process occurs in the kidney in the proximal tubule. At this stage, it is the real McCoy—vitamin D. Here are the steps in sequence so you can remember them:

Vitamin D and calcium regulation

The metabolically active form of vitamin D is released into circulation and binds to a carrier protein in the plasma—vitamin D binding protein (VDBP)—which transports it to various target organs. To have biological effects, the hormonally active form of vitamin D must next bind to a receptor—vitamin D receptor (VDR)—located in the nuclei of target cells. After binding to the VDR, it acts as a transcription factor that modulates the gene expressions that are involved with calcium absorption in the intestine. The vitamin D receptor belongs to the nuclear receptor superfamily of steroid/thyroid hormone receptors, which are expressed by cells in most organs, including the brain, heart, skin, gonads, prostate and breast. It is the vitamin D receptor activation in the intestine, bone, kidney and parathyroid gland cells that leads to the maintenance of calcium and phosphorus levels in the blood, with the assistance of parathyroid hormone and calcitonin, and to the maintenance of bone content.

The VDR is known to be involved in cell proliferation and differentiation. Vitamin D also affects the immune system, and VDR is expressed in several white blood cells, including monocytes and activated T and B cells.

Calcium is a tightly regulated substance. The concentration in the blood plasma must be held constantly at 10 mg/100 mL of the total calcium. Many of life’s essential functions depend on calcium being constantly available. A few of these include signal transmission in nerves, the contraction and relaxation of muscle tissue, exocrine secretion, blood-clotting, cellular adhesion and the construction of an entire skeleton. To maintain calcium at a constant level, a process called calcium homeostasis occurs, which is a very complex task that involves many hormones, some of which are unknown. Managing the calcium of plasma is the most important role of vitamin D in the endocrine system, a task it shares with the parathyroid hormone and a substance called calcitonin. See Figure 2.

The parathyroid gland is the calcium-sensing organ. A slight drop in the calcium plasma level, known as hypocalcemia, will cause the parathyroid gland to react within seconds to secrete parathyroid hormone (PTH).1 This is followed by a sequence of events that mobilizes calcium to restore the normal plasma level. The calcium receptor in the parathyroid gland is well-known and appears to act to facilitate the secretion of PTH, which has a very short lifespan in plasma, in minutes, if not seconds.2

There is a receptor for the PTH found throughout the length of the nephron of the kidney, in the osteoblasts (bone cells that lay down calcium), but not the osteoclasts (bone cells that absorb calcium) of the skeleton. In the kidney, the PTH blocks reabsorption of the phosphate, causing a loss of phosphate. Several hormonal functions of PTH relate to its role in the formation of vitamin D in the kidney. The following references will expand these facts for those who are interested.3–5

Calcitonin

Calcitonin is a 34-amino acid peptide hormone that is a major compound for lowering serum calcium by its action on the skeleton. Increased levels of calcium, or hypercalcemia, cause calcification of soft tissues, such as the kidney, heart, aorta and intestine, resulting in organ failure and death. The deactivation of the parathyroid gland is an important step to control high calcium levels. When calcium levels are high, it triggers the activation of the parafollicular cell (C cells) of the thyroid to secrete the hormone calcitonin. Also, calcitonin acts on osteoclasts and osteocytes to reduce the calcium-mobilizing activity and deactivation of the calcium coming from the skeleton. Although other actions of calcitonin have been described in both the kidney and intestine, the most important is the regulation of serum calcium that occurs in the skeleton.6

Vitamin D, the immunomodulator

Vitamin D, when activated, has many important roles in helping the immune system function well. It does this by binding to nuclear VDR, present in most immune cell types—both innate and adaptive. The VDR is expressed in monocytes and in activated macrophages, dendritic cells<sup>b</sup> (DC), natural killer (NK) cells, and T and B cells. This means that vitamin D has potent antiproliferative, pro-differentiative and immunomodulatory functions,including both immune-enhancing and immuno-suppressive effects.7

When vitamin D binds to the receptor, it increases the activity of NK cells and enhances the phagocytic activity of macrophages. Vitamin D deficiency tends to increase the risk of infections, such as influenza and tuberculosis.8

For example, a study in 1997 showed that Ethiopian children with rickets, or vitamin D deficiency, were 13 times more likely to get pneumonia than children without rickets.9 Vitamin D, when activated, has been shown to affect DC maturation, differentiation and migration, which relates to inhibition of DC-dependent T cell activation, the final result being immunosuppression.10

In 2006, the Journal of the American Medical Association published a report showing a potential link between vitamin D deficiency and the onset of multiple sclerosis. The authors suggest that this is due to the immune-response suppression properties of vitamin D. It is possible that vitamin D is required to activate an immune response that triggers the mechanism for individuals to recognize self from nonself.11

Cancer prevention

In tissue cultures, the vitamin D hormone has been found to induce the death of cancer cells. This positive activity of vitamin D is believed to result from its action as a nuclear transcription factor that regulates cell growth, differentiation and programmed cell death, called apoptosis. In addition, vitamin D appears to regulate many cellular mechanisms central to the development of cancer, and again these actions appear to be through the expression of the vitamin D receptor.

There is an increasing body of research that supports the concept that the active form of vitamin D may provide significant protective effects against cancer.12 A study in 2006 found that taking vitamin D (400 international units [IU<sup>c</sup>] per day) cuts the risk of pancreatic cancer by 43% in a test of more than 120,000 people.13 A study involving 1,200 women, published in June 2007, reports that vitamin D at 1,100 IU/day resulted in a 60% reduction in cancer incidence during a four-year clinical trial.14

Vitamin D and the skin

Insight into possible new functions of 1,25-(OH)2D3 beyond the regulation of calcium and phosphorus resulted from studies involving the cellular localization of 1,25-(OH)2D3. The result of this study illustrated that 1,25-(OH)2D3 was localized in the nuclei of the cells in the small intestine, the distal renal tubule of the kidney, the osteoblasts of bone and in tissues not previously considered targets of vitamin D action. In addition, 1,25-(OH)2D3 has been found in the nuclei of the islet cells of the pancreas, keratinocytes of skin, ovarian tissue, mammary epithelium, epithelial cells of the epididymis, neuronal tissue, promyelocytes, macrophages and T lymphocytes—these cells are all blood-derived.

What these new findings mean is that the possibility of 1,25-(OH)2D3 to carry out not only many other functions, but also in cells not even considered as target cells of vitamin D. Other studies followed that showed the vitamin D receptor to be present in the skin,15 thymus and ovarian cells. It is now evident that the vitamin D hormone carries out specific functions in many of these tissues, and many other functions remain to be discovered.

The role of vitamin D in the keratinocyte remains a fascinating but controversial topic among scientists studying vitamin D. Although it is now accepted that the keratinocytes are induced to differentiate by the in vitro addition of 1,25-(OH)2D3, the absence of vitamin D does not cause a problem with keratinocyte differentiation. Possibly, the differentiation may be triggered by more than one system, which would explain why the skin functions normally with a vitamin D deficiency. However, the situation becomes more complex because, together with the differentiation of the keratinocyte, there comes an inhibition of keratinocyte proliferation. From this observation, a new treatment for psoriasis was developed.<sup>16</sup> New products called “analogues” of vitamin D have shown a significant positive effect against psoriasis with as many as 70% of patients responding to this treatment. Exactly how 1,25-(OH)2D3 induces differentiation of the keratinocyte and inhibits proliferation is the big unknown and remains to be investigated.

It has been proposed that the keratinocyte functions to serve in a paracrine<sup>d</sup> manner to stimulate the differentiation of other keratinocytes. However, it remains to be explained how vitamin D-deficient animals are able to obtain keratinocytes that are mature and function normally. Thus, the role of 1,25-(OH)2D3 in the keratinocyte under normal physiological circumstances is unknown, and certainly, the idea that the keratinocyte in vivo is able to produce 1,25-(OH)2D3 is not universally accepted. Much research remains to be done before the last word is written.

Vitamin D and skin peptides

Sunlight exposure, except for promoting vitamin formation, is generally bad. It was surprising to find that the effect of sunlight on the human innate immune system could be of great clinical significance. It has been reported that a UVB band of ultraviolet light stimulates expression of a substance known as hCAP-18, which is the precursor of the antimicrobial peptide known as LL-37 in human skin.<sup>17</sup> The skin of eight individuals was exposed to a minimal erythematous dose<sup>e</sup> of both UVA (300–400 nm) and UVB (280–315 nm) wavelengths on different body sites and was biopsied 24 hours before and after exposure. The skin samples were analyzed for the quantity of hCAP-18 messenger RNA (mRNA) and the amount of vitamin D receptor protein present. The scientists reported that each of the subjects exposed to UVB light showed an increase in both the amount of hCAP-18 mRNA and vitamin D receptor in their skin.

There was a positive correlation between the amount of skin antimicrobial peptide and the amount of vitamin D receptor present, suggesting that the level of hCAP-18 was directly influenced by the increase in vitamin D receptor levels. This is very interesting because it shows a new function of vitamin D. There was no effect shown with UVA exposure in this study, nor was there any correlation of effect with skin type.

How much do we need?

Vitamin D is produced by the human body when it is exposed to direct sunlight, but big variations exist. The season, geographical latitude, time of day, cloud cover and sunscreen use all affect UV ray exposure and thus vitamin D synthesis in the skin. I strongly recommend that you do have limited sun exposure as a good source of vitamin D. Extra vitamin D is also recommended for older adults and people with dark skin. Currently, scientists recommend 25 μg (1,000 IU) of vitamin Dbe consumed daily to maintain adequate blood concentrations of 25-hydroxy vitamin D.18

Summary

Vitamin D is formed in the skin from cholecalciferol produced by the action of UVB light on an earlier precursor. This compound undergoes changes that require modification by the liver and kidneys to produce the active form know as 1 α, 25 dihydroxy D3, or true vitamin D. Vitamin D is a hormone with receptor and transcription activity. It is known that in some cancers vitamin D is preventive. In the immune system, vitamin D has potent antiproliferative, pro-differentiative and immunomodulatory functions, including both immune-enhancing and immunosuppressive effects. In the skin, it has several functions that control epidermal proliferation by action on the keratinocytes. In addition, when the skin is irradiated with UVB, vitamin D stimulates the production of an epidermal peptide that is antimicrobial.

With all of these functions, the single most important one is the regulation of the level of calcium in the blood. It does this by its action on the parathyroid hormone and its role in calcitonin regulation. In the September 2009 issue of Skin Inc. magazine, the final part of this series will explore vitamin E.

Footnotes

a The UV index is a scale with higher values representing the risk level of skin damage due to UV exposure. A value of 0 corresponds to zero UV irradiation, such as occurs during nighttime. An index of 10 corresponds roughly to midday sun and a clear sky. The numbers are related to the amount of UV radiation reaching the surface of the Earth, measured in watts per square meter of surface.

b Dendritic cells (DC) are immune cells that form part of the mammalian immune system. Their main function is to process antigen material and present it on the surface to other cells of the immune system, thus functioning as antigen-presenting cells. DCs are present in small quantities in tissues that are in contact with the external environment, mainly the skin (where there is a specialized DC type called Langerhans cells) and the inner lining of the nose, lungs, stomach and intestines.

c One IU equals 25 nanograms.

d Paracrine means a type of cell stimulation that is initiated by other cells close by, as opposed to endocrine stimulation that occurs from afar.

e Known as the MED, it is the lowest radiation energy of UVB that will make the skin appear red.